Physical properties of young (3.5 Ma) oceanic crust from ...afisher/CVpubs/pubs/BartetzkoFisherJGR_2008small.pdf · and water. Much of this ridge-flank circulation occurs at ... fluids

Physical properties of young (3.5 Ma) oceanic crust from the eastern

flank of the Juan de Fuca Ridge: Comparison of wireline and core

measurements with global data

A. Bartetzko1 and A. T. Fisher2

Received 13 July 2007; revised 7 December 2007; accepted 30 January 2008; published 29 May 2008.

[1] We compile and compare petrophysical data from the upper oceanic crust in HoleU1301B, on 3.5 Ma seafloor on the eastern flank of the Juan de Fuca Ridge.Measurements include core-scale (lab-based) and in situ (wireline) measurements offormation porosity, electrical resistivity, bulk density, P wave velocity, and gamma rayemission. A comparison between Hole U1301B data and those from other sites, ranging inage from 5.9 to 167 Ma, demonstrates some important differences. Hole U1301B samplestend to have lower bulk density values for a given porosity than do other youngcrustal sites. Hole U1301B samples also tend to have higher values of formation factor(the ratio of fluid conductivity to saturated rock conductivity), which may indicatedevelopment of tortuous microporosity that is more consistent with samples from olderseafloor. Evaluation of global trends of petrophysical properties versus crustal age indicatesthat Hole U1301B in situ data is consistent with global trends, but core-scale P wavevelocity and bulk density values fall below global trend values predicted for young crust.This last observation is likely related to basement topography and the regional exposure,until relatively recently, of basement outcrops around Site U1301. Until most of theexposed basement in this area was buried, hydrothermal circulation extracted much of thelithospheric heat, kept basement fluid temperatures low, and limited the rate and extent ofalteration; a subsequent period of higher-temperature fluid circulation with morerestricted and less oxidative conditions resulted in enhanced and pervasive alteration ofcrustal rocks.

Citation: Bartetzko, A., and A. T. Fisher (2008), Physical properties of young (3.5 Ma) oceanic crust from the eastern flank of

the Juan de Fuca Ridge: Comparison of wireline and core measurements with global data, J. Geophys. Res., 113, B05105,

doi:10.1029/2007JB005268.

1. Introduction

[2] Interactions between the lithosphere and hydrother-mal fluids on the flanks of mid-ocean ridges result inconsiderable modifications in the compositions of crustand water. Much of this ridge-flank circulation occurs atlow temperatures, leading to relatively slow reaction rates,but the magnitude and duration of ridge-flank hydrothermalalteration leads to large changes in the physical properties ofthe crust (particularly porosity, seismic velocity, and per-meability) mainly through the formation of secondaryminerals that replace original grains and fill void space[e.g., Houtz and Ewing, 1976; Alt et al., 1986; Jacobson,1992; Alt, 1995; Carlson, 1998; Grevemeyer et al., 1999;Becker and Fisher, 2000; Jarrard et al., 2003; Johnson andPruis, 2003]. The oceanic crust accumulates a thickening

layer of relatively low-permeability sediments as it movesaway from the ridge, which eventually limits the freeexchange between ocean bottom water and the basalticoceanic crust. The oceanic crust continues to lose a mea-surable fraction of lithospheric heat to a mean age of 65 Ma,but some seafloor areas lose heat advectively to muchgreater ages, and heat can be redistributed regionally bycontinuing circulation to ages over 100 Ma [e.g., Parsonsand Sclater, 1977; Stein and Stein, 1992; Von Herzen,2004]. Factors that contribute to variations in the distribu-tion and intensity of ridge-flank hydrothermal circulationinclude the nature of crustal permeability, the properties ofaccumulating sediments, the extent of off-axis volcanic andtectonic activity, and the characteristics of basement topog-raphy, particularly the distribution of outcrops [e.g., Daviset al., 1992; Macdonald et al., 1996; Villinger et al., 2002;Fisher et al., 2003a, 2003b; Spinelli et al., 2004a]. The lastfactor in the list above is increasingly recognized to beimportant for facilitating the exchange of low-temperaturefluids between the crust and ocean because the forcesdriving ridge-flank circulation are modest, the hydraulicimpedance of ridge-flank sediments is great, and seamountsand other basement outcrops provide highly transmissive

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1Research Center Ocean Margins, University of Bremen, Bremen,Germany.

2Earth and Planetary Sciences Department and Institute for Geophysicsand Planetary Physics, University of California, Santa Cruz, California, USA.

Copyright 2008 by the American Geophysical Union.0148-0227/08/2007JB005268$09.00

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http://dx.doi.org/10.1029/2007JB005268

hydrothermal pathways on a global basis [e.g., Wessel,2001; Harris et al., 2004; Fisher, 2005].

[3] The seafloor east of the Juan de Fuca Ridge (JFR;Figure 1) has become one of the best-studied ridge flanks inthe past three decades, having been the focus of numerousship, submersible, and drilling programs. Rapid turbiditedeposition during Pleistocene sea level low-stands hasburied much of the oceanic basement on the eastern flankof the JFR, except close to the active ridge and where a fewseamounts and other basement outcrops penetrate regionallythick sediments [Davis et al., 1992; Underwood et al., 2005;Zuhlsdorff et al., 2005]. Nearly complete burial of this ridgeat an unusually young age makes this an ideal area to studyridge flank hydrothermal processes, where the rates andpatterns of circulation are intense and particularly well-constrained [e.g., Davis et al., 1997b; Fisher et al., 2003a;Hutnak et al., 2006].

[4] Ocean Drilling Program (ODP) Leg 168 drilled atransect of 10 sites at distances of 23 to 103 km from theJFR, on 0.9–3.6 Ma seafloor [Davis et al., 1997a](Figure 1). Because the focus of ODP Leg 168 was onshallow processes and lateral gradients, basement penetra-tion and coring was limited to the uppermost 50 m of crust,and there was no wireline logging in basement. The easternend of the Leg 168 transect of sites comprised an area ofrough basement topography, where the seafloor is generallyflat, but the depth to basement varies from 250 to 600 m.Integrated Ocean Drilling Program (IODP) Expedition 301

returned to this area of rough basement and drilled two newbasement boreholes at Site U1301 on 3.5 Ma seafloor[Fisher et al., 2005]. Hole U1301B penetrated 265 m ofturbidites and hemipelagic sediments and 318 m of base-ment. The upper 86 m of basement in Hole U1301B weredrilled without coring and cased, in an effort to maintaingood drilling and coring conditions at greater depths, and tofacilitate emplacement of long-term, subseafloor observato-ries to be used for single- and cross-hole hydrogeologicexperiments [Fisher et al., 2005]. The interval from 86 to318 m into basement was cored and geophysically loggedusing wireline tools, providing the samples and data de-scribed in this study. Although the depth extent of coringand logging in Hole U1301B is modest, there are relativelyfew basement holes that have been drilled, cored and loggedto greater depths during the last forty years of scientificocean drilling. In addition, Hole U1301B was drilled intoyounger seafloor than other basement holes from whichsamples and data can be compared.

[5] Alteration studies carried out on Leg 168 samplesshow that low-temperature alteration within the uppermost50 m of basement is ubiquitous. Alteration intensityincreases along the ridge flank from west to east togetherwith basement temperature and age [Hunter et al., 1999;Marescotti et al., 2000; Porter et al., 2000]. The secondarymineral assemblages indicate that alteration of the upper-most oceanic crust began in an open system under oxidativeconditions with interaction between unmodified to slightlymodified seawater and basaltic crust, producing alterationhalos along fractures. The regime changed to more restrictedconditions with strongly modified seawater-derived fluids,and non-oxidative alteration conditions resulting in a perva-sive alteration and the sealing of veins and void space withsaponite and carbonate [Hunter et al., 1999]. Alterationstudies showed that the rocks did not experience temper-atures higher than those at present (15–64�C) [Marescotti etal., 2000].

[6] In this study, we compare the physical properties ofbasaltic basement measured on core samples in the labora-tory to those measured under in situ conditions withborehole (wireline) tools. We also investigate the influenceof low-temperature hydrothermal circulation on the physicalproperties of young oceanic crust through comparison ofdata from Hole U1301B with data from older seafloorboreholes. Most of the laboratory measurements of coresamples were completed shipboard during IODP Expedition301 but additional measurements were completed onshoreafter the expedition.

2. Drilling Results From Hole U1301B

[7] Core recovery in Hole U1301B basement averaged30%. Basement consists of: (1) basalt-hyaloclastite breccia,(2) aphyric to highly phyric pillow basalt, and (3) massivebasalt [Fisher et al., 2005]. Pillow basalt was the mostabundant rock type (Figure 2). The pillows have dominantlyhypocrystalline textures with a glassy to microcrystallinegroundmass and are sparsely to highly plagioclase ± clino-pyroxene ± olivine phyric. The pillows are sparsely vesic-ular, containing 1–5% round gas vesicles. Massive basaltsconsist of continuous sections of up to 4.5 m of similarlithology. The massive basalts are very similar to the

Figure 1. (a) Location of ODP Leg 168 and IODPExpedition 301 drill sites; (b) schematic profile through theeastern flank of Juan de Fuca Ridge and location of Leg 168and Expedition 301 drill sites (modified after Davis et al.[1997a]).

B05105 BARTETZKO AND FISHER: PHYSICAL PROPERTIES OF OCEANIC CRUST

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sparsely phyric pillow basalts. They are sparsely to highlyvesicular, with an average of 1%–5% round gas vesicles, upto 3 mm in diameter. Massive unit 6 has a distinct 20 cmwide band in its center with 15% vesicles [Fisher et al.,2005].

[8] Geochemical analysis of basalt samples indicates thatthey are normally depleted mid ocean-ridge basalt (MORB).Basement rocks are mostly slightly to moderately altered,with secondary minerals (1) lining or filling vesicles andcavities, (2) filling fractures and veins, (3) replacing phe-nocrysts, or (4) replacing interstitial mesostasis and glass.Thin section observations indicate that the degree of alter-ation varies between 5% and 25%, and up to �60% inhyaloclastites. Clay minerals are the most abundant second-ary minerals [Fisher et al., 2005].

3. Petrophysical Data Collection Methods

3.1. Shipboard Core Measurements

[9] Sampling strategy and routines for laboratory coremeasurements and major results of these analyses aredescribed in detail by Fisher et al. [2005] and summarized

below. Laboratory methods followed standard ODP andIODP techniques [Blum, 1997]. P wave velocity wasmeasured in three directions (x, y, and z) on 106 cubicsamples after samples were cut and saturated with seawater.Only measurements in the vertical (z) direction are pre-sented in this study. The density and porosity of most Pwave velocity samples were also determined.

[10] Moisture and density properties were determined on83 discrete samples on the basis of wet and dry samplemasses and volumes. Moisture content was determined bymeasuring the sample mass before and after removal ofinterstitial pore fluid through oven-drying for 24 h at 100–110�C. Dry volumes were determined with a gas pycnom-eter. Because samples are initially saturated with seawater,corrections for the mass and volume of salt were applied.These measurements do not require specific sample geom-etry, allowing moisture content and density to be determinedon samples that were not properly shaped for measurementsof P wave velocity (e.g., hyaloclastites). Shipboard coredata (those presented herein and numerous additional anal-yses) are available from the IODP database (http://iodp.tamu.edu/database).

Figure 2. Composite log of Hole U1301B. The left panels show the core recovery and lithostratigraphyfrom cores [from Fisher et al., 2005], the right panels give a selection of downhole logs (solid lines) andphysical properties measured on core samples (symbols). Discrepancies between core data and wirelinelogging data are mostly explained by the strong fracturing of the formation and different samplingvolume of core samples compared to wireline measurements. Basalt lava = basalt that could not beunequivocally classified.


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3.2. Onshore Core Measurements

[11] Twenty basalt minicores of 2.54 cm in diameter and2–3 cm in length were taken during IODP Expedition 301for post-cruise measurements of electrical resistivity. Sam-ples were taken immediately after splitting of the cores inorder to prevent evaporation of pore fluids and thus precip-itation of salt in the pore space. Samples were stored inseawater until the measurements were carried out. Measur-ing routines are principally those as described by Bartetzkoet al. [2006]. Two current steel electrodes were placed onthe faces of the minicore. Two thin copper wires werewrapped around the sample (distance between the wires1 cm) to measure electrical potential. A constant current Iof 100 nA with a frequency of 8.3 Hz was introducedthrough the samples and the potential difference U betweenthe copper wires was measured. At least five measurementswere made on each sample and averaged. After electricalmeasurements were finished, density and porosity weredetermined following the same methods used for shipboardanalyses.

[12] There are data from fewer locations that can becompared to results from Hole U1301B than are availablefor other petrophysical properties because electrical resis-tivity is not a standard shipboard measurement, and hasbeen quantified in only a few shore-based studies ofbasement rocks.

3.3. Wireline Logging

[13] Wireline logging operations carried out in HoleU1301B are documented by Fisher et al. [2005, andreferences therein], and tools are described briefly in thissection. All wireline logging data shown in this study areavailable from IDOP logging database (http://iodp.ldeo.columbia.edu/DATA).

[14] The caliper log is a mechanical measurement ofborehole diameter, useful for assessing the reliability of otherwireline logs, but also a semi-quantitative indicator of for-mation brecciation and fracturing, both of which can lead to

borehole enlargements and washouts. Electrical resistivitywas measured with an induction array tool, which uses ahigh-frequency alternating current to induce current flow inthe formation. Wireline bulk density data were collected witha litho-density tool, which uses an active gamma ray sourceand a detector to record backscattered gamma rays, theintensity of which is proportional to formation density. Thesource and detector are pressed against the borehole wallduring measurement, so irregular and washed-out intervalsgenerally produce lower-quality data. Apparent formationporosity was measured with a tool that emits fast neutronsthat are slowed by collision with particles of similar mass,mainly hydrogen atoms, which are often associated withwater in pore spaces. P wave velocity data were acquired witha tool having transmission and receiver tool sections sepa-rated by 3–4 m, and using frequencies of 8–30 kHz. Naturalgamma ray intensity was measured by bismuth germanatescintillation.

4. Results

4.1. Comparison Between Physical Properties FromCore Samples and Wireline Logs

[15] Petrophysical properties measured on core samples(shipboard and onshore) and by wireline logging are plottedtogether in Figure 2. Results of the shore-based coremeasurements are listed in Table 1. Density and porositymeasured on core samples shipboard and onshore agreewell. Bulk density varies between 1.86 and 3.03 g/cm3

(average value 2.76 ± 0.09 g/cm3) and porosity variesbetween 0.02 and 0.30 (average value 0.06 ± 0.03). Graindensity is 2.23 to 3.11 g/cm3 (average value 2.87 ± 0.06 g/cm3).P wave velocity from core samples (measured shipboardonly) ranges from 3.9 to 5.9 km/s (average value 5.16 ±0.32 km/s), whereas electrical resistivity (measured onshoreonly) is 15 to 192 Wm. Formation factor (ratio of fluidconductivity to saturated rock conductivity) ranges from 73to 983.

Table 1. List of Samples Analyzed in This Study and Results of Petrophysical Measurements (Rseawater = Resistivity of Samples

Saturated With Seawater)a

SampleDepth,mbsf Unit

Rseawater,Wm

Bulk Density,g/cm3

Grain Density,g/cm3

Porosity(fractional)

FormationFactor ± std. dev.

2R-1W, 67–70 357.77 1C 58 2.71 2.83 0.07 287 ± 553R-1W, 136–139 362.46 1C 145 2.72 2.84 0.06 699 ± 954R-3W, 35–37 369.74 1C 114 2.83 2.93 0.05 543 ± 535R-2W, 75–78 378.55 1C 111 2.77 2.88 0.06 553 ± 1066R-1W, 83–85,5 386.83 1C 32 2.60 2.78 0.10 160 ± 3011R-1W, 32–35 424.72 1C 125 2.74 2.84 0.06 614 ± 7011R-1W, 116–119 425.56 2A 81 2.81 2.90 0.05 400 ± 6112R-1W, 107,5–110,5 429.98 2A 132 2.83 2.91 0.04 645 ± 8413R-2W, 22–25 431.55 2B 61 2.63 2.78 0.08 309 ± 5014R-1W, 23,5–26,5 434.24 4 172 2.66 2.78 0.07 872 ± 11115R-1W, 139,5–142,5 445.00 4B 48 2.86 2.98 0.06 241 ± 3618R-2W, 84–86,5 473.14 6 15 2.62 2.94 0.16 73 ± 618R-2W, 122–124,5 473.52 6 123 2.91 2.96 0.03 631 ± 12621R-4W, 45–48 494.94 7B 145 2.77 2.87 0.05 742 ± 15423R-2W, 118,5–121,5 502.55 7B 159 2.88 2.95 0.04 785 ± 13825R-2W, 12,5–15,5 511.04 7B 65 2.75 2.89 0.07 325 ± 3632R-1W, 115–118 551.15 7C 100 2.73 2.84 0.06 536 ± 13233R-2W, 101,5–104,5 556.52 7C 94 2.72 2.82 0.06 474 ± 6335R-2W, 125–128 566.32 8B 192 2.87 2.93 0.04 983 ± 18136R-1W, 142–145 574.62 8C 86 2.72 2.84 0.07 441 ± 68

aStd. dev: standard deviation.


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[16] Electrical resistivity values measured on core samplesare generally higher than those measured by wireline logging(Figure 2). Bulk density values from core samples correspondto the highest density values from wireline logging, andporosity data from core samples are generally lower thanestimates based on (neutron) porosity logs. Within the shortdepth interval where wireline P wave velocity logs areavailable, core and wireline data agree well and the meancore value is also close to the 5.0 km/s determined by checkshot vertical seismic profiling [Fisher et al., 2005].

[17] Petrophysical properties determined from core sam-ples and by wireline logging commonly differ for a varietyof reasons, several of which are specific to fractured igneousrocks of the upper oceanic crust [e.g., Broglia and Moos,1988; Jarrad et al., 2003]. First, core recovery in the upperoceanic crust is notoriously low and biased; recovery formassive units is favored, and regions having the mostintense fracturing tend to be underrepresented. Second, coresamples allow assessment of petrophysical properties at aspatial scale of centimeters, whereas wireline logs (of thetype described in this study) resolves properties at scales ofdecimeters to meters. The latter may include fractures,breccia zones, and regions of intense alteration. Many ofthe rare breccia samples recovered on IODP Expedition 301were dedicated to microbiological analyses [Fisher et al.,2005], but core measurements were completed on onebreccia sample, and the results compare favorably to thosefrom geophysical logging (Figure 2, data near 350 m belowseafloor - mbsf).

[18] The discrepancy between petrophysical propertiesdetermined on core samples and by wireline logging ismost apparent in Hole U1301B above 465 mbsf (200 msub-basement; msb), where the caliper log shows commonenlargements of the borehole. Logging tools that requirecontact with the borehole wall (including bulk density andneutron porosity) often lost contact with the borehole wallwithin this interval (particularly between 390–410 mbsf),and data from these instruments are considered to be lessreliable than at greater depths. However, even at depthswhere the bulk density and neutron porosity logs cannotinterpreted quantitatively, comparison with other wirelinerecords (caliper, electrical resistivity, P wave velocity)suggests a consistent formation response indicative offracturing, brecciation, and elevated porosity, particularlyin the upper 100 m of the logged interval (Figure 2).

[19] Throughout the logged interval, the bulk density ofcore samples is consistent with the highest values deter-mined in situ (with the exception of the single brecciasample described earlier). Above 465 mbsf, intervals ofhigher and lower bulk density alternate with a typicalspacing of ten to several tens of meters (Figure 2). Below465 mbsf, the typical bulk density determined by wirelinelogging is greater, and the low-density intervals are thinnerthan observed higher in the section.

[20] Although a similar pattern is apparent in core poros-ity and wireline neutron porosity data, the latter should beviewed cautiously. Porosity measurements made on coresamples are based on simple (relatively direct) measure-ments of mass and volume, subject to the sampling biasdescribed earlier, whereas neutron porosity measurementsare based on the interaction between neutrons emitted fromthe logging tool and atoms in the formation. In clay-free

sediments saturated with water this interaction dependsmainly on hydrogen atoms in pore fluids. In igneous andmetamorphic rocks, the formation – tool response can beinfluenced by the presence of hydrous minerals (particularlysecondary phases), neutron absorbers such as chlorine orgadolinium in the rock or the pore fluid/drilling mud (e.g.,seawater), and the higher matrix density of the rocks [e.g.,Lysne, 1989; Broglia and Ellis, 1990; Harvey and Brewer,2005]. Although sophisticated logging tools minimize forsome of these effects and provide neutron porosity valuescloser to porosities measured on core samples, neutronporosity values still overestimate porosity in the upperoceanic crust. We also calculated an ‘‘apparent porosity’’log using the bulk density log and a range of mean graindensity values, but this log was only marginally differentfrom the neutron porosity log. Washed out intervals thatyielded spurious (high) neutron porosities also yieldedspurious apparent porosities. For this reason, the wirelineneutron porosity data are not interpreted quantitatively inthis study.

[21] Electrical resistivity measured on cores is generallyhigher than electrical resistivity from the same depth inter-val measured by wireline logging. In addition to thedifferent measurement scale inherent in the two methods,there are also differences in the measurement principlesapplied in this case. Core samples were measured at lowfrequency (8.3 Hz) whereas the wireline logging tool usedduring Expedition 301 was an induction tool operating athigh frequency (26.3 kHz) [Fisher et al., 2005]. The use ofdifferent frequencies may have an effect on electricalresistivity, however, the equipment used to measure electri-cal resistivity on core samples in this study is not suitable tomeasure electrical resistivity as a function of frequency.

4.2. Relationship Between Density and P Wave Velocity

[22] Figure 3a shows a crossplot of bulk density versus Pwave velocity from core measurements for the differentlithological units recovered from Hole U1301B. P wavevelocity increases with bulk density, consistent with varia-tions in porosity. There are no systematic differences in thegeneral bulk density versus P wave velocity relationship onthe basis of lava morphology although two samples frommassive unit 6 show surprisingly low density and P wavevelocity values. These samples are highly vesicular, up to15% by volume. Figure 3a shows two regression functionsfor the Hole U1301B data set based on linear and expo-nential fits to the data, with virtually identical coefficients ofdetermination (r2 = 0.48 and r2 = 0.47, respectively).

[23] Figure 3b compares the Hole U1301B data setwith data from other boreholes drilled into oceanic crust(Table 2). P wave velocity increases with increasingdensity in these other data sets (both considered individuallyand in aggregate), but the Hole U1301B data cluster tendstoward lower P wave velocity and density values. For theglobal data sets, linear and exponential regressions fit aboutequally well (coefficients of determination r2 = 0.55 and0.53). The regression functions for Hole U1301B indicate amore gradual increase in P wave velocity for the sameincrease in density as the global regression functions. Boththe data set for Hole U1301B and the global data setanalyzed in this study give a smaller increase in P wavevelocity for the same increase in density than the data set


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analyzed by Christensen and Salisbury [1975], whichincludes basalt samples from Deep Sea Drilling Projectboreholes.

4.3. Relationship Between Porosity and Density

[24] Figure 4 shows crossplots of bulk density versusporosity measured on core samples. As expected, bulkdensity decreases with increasing porosity. As seen withthe P wave velocity versus bulk density relationship,there is no systematic variation on the basis of lavamorphology. Breccia samples have lower bulk densityand considerably higher porosity than the more massivesamples, and the vesicular samples from massive unit 6have higher porosities than most of the other samples.Figure 5 compares bulk density versus porosity fromHole U1301B samples to those from the global data set(Table 2). Most porosities are between 0.015 and 0.08and most of the bulk densities are between 2.8 and2.95 g/cm3 and correspond to grain density values between2.9 and 3.0 g/cm3. The global data follow a relativelyconsistent trend for porosities �0.07, but there is consid-

erable scatter and a weaker bulk density versus porositytrend at higher porosities.

[25] Hole U1301B samples tend toward lower bulk den-sity and higher porosity than the majority of the samplesfrom other holes. More than half of the samples from HoleU1301B show grain density values below 2.9 g/cm3. Incontrast, Hole 504B samples tend to have higher bulkdensity for a given porosity than samples from other holes(Figure 5), suggesting higher grain densities. Measuredgrain densities of Hole U1301B samples vary between2.2 and 3.1 g/cm3, but most samples are between 2.7 and3.0 g/cm3 [Fisher et al., 2005]. The grain density of HoleU1301B samples generally decreases with increasing po-rosity. This may result from enhanced water-rock interactionand alteration, favored by greater rock surface area and/orhigher permeability [e.g., Jarrard et al., 2003].

4.4. Relationship Between Formation Factor andPorosity

[26] Although the formation factor, F, is commonlydetermined as the ratio of fluid conductivity to saturated

Figure 3. Bulk density versus P wave velocity measured on core samples from Hole U1301B for thedifferent lithological units shown in Figure 2 (left) and in comparison with core data from other boreholesin basaltic oceanic basement (right). Regression formulae for density in g/cm3 and P wave velocity in km/s.


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rock conductivity, it can also be described as the ratio oftortuosity, a, to porosity, f [Kan and Sen, 1987]:

F ¼ a=f ð1Þ

[27] Tortuosity is a geometric description that takes intoaccount the indirect paths around mineral grains, sometimesdefined as the square of the ratio of pore path length tosample thickness. Tortuosity may also account for variationsin pore cross-sectional area and the connectivity of the porespace [e.g., Gueguen and Palciauskas, 1994].

[28] The formation factor of core samples from HoleU1301B decreases with increasing porosity, as with otherpetrophysical properties, there is no separation of units onthe basis of lava morphology (Figure 6a). Application ofequation (1) to Hole U1301B data suggests tortuosity in therange of 10–40. A comparison of Hole U1301B formationfactor and porosity data with the global data set (Figure 6b)places Hole U1301B samples between those from Holes504B and 896A (5.9 Ma seafloor south of the Costa RicaRift) and those from Hole 801C (167 Ma seafloor of

Pigafetta Basin, West Pacific). The apparent tortuosity ofHole U1301B samples is higher than for the samples fromHoles 504B and 896A, perhaps indicating higher micropo-rosity in Hole U1301B samples. Apparent tortuosity valuesare higher still in samples from Hole 801C, drilled intosome of the oldest in situ oceanic crust.

5. Comparison Between Hole U1301BPetrophysical Data and Global Trends

[29] Several studies have evaluated age-dependentchanges in physical properties of the oceanic crust. Houtzand Ewing [1976] showed that seismic velocities increasewith increasing age within the uppermost layer of thebasaltic crust. P wave velocities, electrical resistivities andtotal gamma ray measured in situ using downhole wirelinemeasurements also tend to increase with basement age[Bartetzko, 2005]. However, velocities and densities mea-sured on drill cores often decrease with age while inter-granular porosity increases [e.g., Johnson and Semyan,1994; Johnston and Christensen, 1997; Jarrard et al.,2003]. These changes in physical properties with the age

Table 2. Overview of Deep Boreholes Drilled Into Basaltic Oceanic Crust

Hole LocationBasementAge, Ma

Thickness ofSedimentCover, m

TotalDepth,mbsf

DepthInterval,mbsf a

DepthInterval, msba References

395A Mid-Atlantic Ridge 7.3 93 664 120–420 27–327 Melson et al. [1979]418A Bermuda Rise (Atlantic) 110 324 868 330–770 6–446 Donnelly et al. [1980]504B Costa Rica Rift (Pacific) 5.9 274.5 2111 280–587 5.5–312.5 Cann et al. [1983]801C West Pacific 167 461.6 935.4 560–810 98.4–348.4 Lancelot et al. [1990], Plank et al. [2000]896A Costa Rica Rift (Pacific) 5.9 179 469 205–400 26–221 Alt et al. [1993]1224F Central tropical N Pacific 46.3 28 174.5 40–137 12–109 Stephen et al. [2003]1256D Eastern Pacific 15 250 1255 280–720 30–470 Wilson et al. [2003], Teagle et al. [2006]1301B Juan de Fuca Ridge (Pacific) 3.5 265 583 360–540 95–275 Fisher et al. [2005]

aDepth interval considered in the present study.

Figure 4. Bulk density versus porosity measured on core samples from Hole U1301B for the differentlithological units shown in Figure 2.


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of the crust are interpreted to result mainly from theinteraction of water and rock during the ridge-flank hydro-thermal circulation. The circulating water causes sealing ofvoids and fractures with secondary minerals, which influ-ences physical properties on a larger scale (seismic andwireline measurements) [e.g., Houtz and Ewing, 1976;Jacobson, 1992; Bartetzko, 2005]. The increase in P wavevelocity and electrical resistivity represents a decrease inporosity, whereas an increase in total gamma ray resultsfrom the incorporation of potassium from seawater intosecondary minerals. On the scale of core samples, basaltalteration is expressed as an increase in intergranular po-rosity and replacement of the original mineralogy by sec-ondary minerals [e.g., Johnson and Semyan, 1994].

[30] Figure 7 shows the relationship between physicalproperties measured on core samples and measured in situ

with basement age. P wave velocity, electrical resistivity, bulkdensity, and total gamma ray measured in situ using wirelinetools increase with basement age, while P wave velocity andbulk density measured on core samples decrease. Propertiesdetermined with wireline data and core data converge forolder crust. In situ properties from Hole U1301B followglobal trends, although wireline P wave velocity and electri-cal resistivity data from Hole U1301B should be treatedcautiously because (a) P wave velocity data are only availablefor the uppermost basement interval of Hole U1301B (85–150 msb) and (b) electrical resistivity was measured bywireline with an induction tool, compared to the galvanictools used in other holes. Total gamma ray from Hole U1301Bis slightly higher than expected from the global trend, butthere is considerable variability in global data and totalgamma ray intensity may be sensitive to both primary

Figure 5. Bulk density versus porosity for different boreholes drilled in basaltic oceanic basement.Solid and dashed lines: lines of equal grain density.

Figure 6. Formation factor versus porosity for different lithological units from Hole U1301B (left) andin comparison to other boreholes drilled into basaltic oceanic basement (right).


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potassium content and crustal alteration [Bartetzko, 2005].Geochemical analyses of fresh volcanic glass, which isconsidered to reflect primary magma composition, are notyet available for Hole U1301B. Therefore the contribution ofprimary potassium to total gamma ray cannot be estimated.

[31] P wave velocity and bulk density core data fromHole U1301B fall below global trends on the basis ofcrustal age (Figure 7). These low P wave velocity and bulkdensity values indicate higher alteration of the core samplesthan expected for young oceanic crust. High values oftortuosity in Hole U1301B samples (Figure 6), comparedto core samples from Hole 504B and 896A, may indicatehigher microporosity in Hole U1301B samples and alsosuggest enhanced alteration.

[32] The trends for the core data plotted in Figure 7 werecalculated excluding Hole U1301B. Including HoleU1301B results in very low correlation coefficients(�0.33 and �0.42 for bulk density and P wave velocity,respectively). We used the global trend for the core datafrom Figure 7 to calculate a theoretical mean value for bulk

density and P wave velocity for core measurements ofsamples from Hole U1301B. These estimated values basedon the global age-trend are considerably higher than valuesobtained from direct testing of core samples. For bulkdensity, the expected value from the age trend is 2.88 g/cm3

(compared to 2.75 g/cm3 from core measurements) andfor P wave velocity the expected value is 5.73 km/s(compared to 5.16 km/s from the core data set). In orderto check the statistical significance of Hole U1301B coreobservations, a t test was applied to compare the mean ofmeasured values of core bulk density and P wave velocityfrom Hole U1301B with values determined from otherbasement holes (Table 3). The t test shows that the meanvalues of bulk density and P wave velocity are significantlydifferent at a confidence level of 99% for Hole U1301Bcompared to other holes drilled into young crust(Holes 504B, 896A, 395A, and 1256D). For holes drilledinto older crust (Holes 1224F, 418A, and 801C), differencesin mean values compared to Hole U1301B are less significant.This result is consistent with the observation in Figure 7, that

Figure 7. Average values (±1 standard deviation) of P wave velocity, electrical resistivity, bulk density,and total gamma ray for wireline logs (black symbols and lines) and core data (P wave velocity, and bulkdensity, grey symbols and lines) plotted versus crustal age. Numbers give correlation coefficients (r).Regression lines for core data were calculated excluding Hole U1301B. The depth interval 390–410 mbsf of Hole U1301B was not included for calculating average values because the hole wasseverely oversized within this interval (Figure 2).


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core properties from Hole U1301B more closely resembleproperties of old crust than to those from young crust.

6. Constraints for the Hydrogeology at Juan deFuca Ridge

[33] Thermal, pressure, and geochemical (pore fluid) datafrom Site U1301 and the surrounding region indicatevigorous regional and local convection of basement fluidsat (relatively high) temperatures of 60–65�C [e.g., Davis etal., 1997b; Wheat et al., 2000; Davis and Becker, 2002;Fisher et al., 2003a, 2005; Spinelli et al., 2004b; Fisher,2005]. The extent of fluid alteration determined fromborehole and warm-seep fluid samples collected nearby atSite 1026 and Baby Bare outcrop suggests that water-rockinteraction is unusually advanced for the age of the crust inthis area [e.g., Mottl et al., 1998; Wheat et al., 2004]. Thusregional hydrogeological studies explain the intense alter-ation observed from samples from Hole U1301B on thecore scale, but may not explain the more modest alterationapparent on the in situ scale.

[34] We explain the behavior in physical properties de-scribed above by the specific geological situation at theeastern flank of Juan de Fuca at the location of the drill site.Site U1301 is located on a buried basement high that wasformed by a combination of tectonically created abyssal-hilltopography and off-axis volcanic construction. The influ-ence of ridge-flank volcanic processes on upper basementtopography near Site 1301 is readily apparent on maps ofbasement relief generated with combined seafloor swath-map and seismic data [e.g., Zuhlsdorff et al., 2005; Hutnaket al., 2006]. Site U1301 is located within 5–16 km ofseveral present-day outcrops, and there was much moreextensive basement exposure in this area in the last 100–500 ka [Hutnak and Fisher, 2007]. Hydrogeologicconditions during this time would have been very similarto those seen today near the western end of the ODP Leg168 drilling transect [Davis et al., 1997a; Elderfield et al.,1999]. Basement alteration and intensity would haveincreased and become more pervasive once rapid Pleisto-cene sedimentation buried most of the exposed basementaround Site U1301.

[35] The current setting at Site U1301 is similar to that atODP Site 896 on the southern flank of the Costa Rica Rift.Hole 896A was drilled into a small basement knoll (140–150 m high, 600–900 m in diameter) that sits atop anabysall hill. This knoll was interpreted as a small volcanic

cone emplaced off-axis [Swift et al., 1998]. Drilling resultsof Hole 896A show that fracturing is stronger and alterationis more pervasive than seen in neighboring Hole 504B [e.g.,Harper and Tartarotti, 1996; Wilkens and Salisbury, 1996;Bartetzko et al., 2002]. Much like the basement surroundingSite U1301, basement at Site 896 was likely one of the lastareas of seafloor to be buried by accumulating sediments.The rocks drilled in Hole 896A show a more widespreadoxidizing alteration and higher water/rock ratios than ob-served at the neighboring Site 504 [e.g., Laverne et al.,1996]. Petrophysical investigations on core samples indicatea change in the mechanical properties of the rocks due tomicrofissuring and/or pervasive alteration, as massivebasalts of 896A have higher porosities than those of Hole504B, and the relation between P wave velocity and densityis different in the two holes. At similar densities, coresamples from Hole 504B have higher P wave velocitiesthan those of Hole 896A [Wilkens and Salisbury, 1996].Similar relationships between bulk density and P wavevelocity were observed for Hole U1301B. The strongfracturing in Hole 896A also resulted in very low electricalresistivity values measured in situ, which are below theglobal trend (Figure 7).

[36] Unlike Hole 896A, Hole U1301B does not penetratewhat used to be a volcanic edifice; it is several kilometersfrom the nearest (former) edifice. Instead, the crustal stra-tigraphy and porosity structure in Hole U1301B is similar insome ways to that from Holes 504B and 395A. Both holeshave an upper, highly porous and permeable interval (upper130 and 300 m of basement, respectively) and a lower-porosity lower interval [e.g., Matthews et al., 1984; Pezardand Anderson, 1989]. Hole U1301B has an uppermostinterval (above 465 mbsf) characterized by poor holeconditions, pervasive washouts, and thick zones of lowbulk density and electrical resistivity in wireline logs, bothindicating high porosity. The deeper (but still upper-crustal)interval is characterized by thinner zones having lowbulk density and electrical resistivity. The entire uppercrustal section around Hole U1301B is highly permeable[Fisher et al., 2008; Becker and Fisher, 2008], but wirelinepetrophysical data clearly distinguish between two regionswithin the upper crust (Figure 2). The high permeability ofthe upper crust at Site U1301B enables circulation of largequantities of fluids through the crust.

[37] Field studies and numerical modeling suggest thatbasement outcrops on ridge flanks become favored sites ofhydrothermal discharge and recharge once most of theregional basement is buried and outcrops provide the fewremaining access points for fluids to enter and exit the crust[e.g., Villinger et al., 2002; Fisher et al., 2003a, 2003b;Hutnak et al., 2006]. Therefore before most of the outcropswere buried around Site U1301, large quantities of watercirculated through the rocks, resulting in a long initialperiod of low-temperature fluid circulation. When muchof regional basement was exposed, hydrothermal conditionsin basement were relatively open, and geochemical con-ditions were oxidative. This initial period was followed by ashorter, more recent period of higher-temperature fluidcirculation (when basement conditions become more re-stricted and were less oxidative). In Hole U1301B, theinitial alteration phase, resulting primarily in the formationof alteration halos rather than the sealing of fractures, was

Table 3. Results of t Test to Determine Statistical Significance of

Differences Between Results From Core Samples From Hole

1301B and Similar Samples From Other Ocean Boreholes in

Basaltic Basementa

504B 896A 395A 1256D 1224F 418A 801C

Bulk density1301B 99.99 99.99 99.99 99.99 99.70 *82.90 98.60

P wave velocity1301B 99.99 99.99 99.99 99.99 88.00* 98.00 94.10*

aNumbers give the confidence level (in percent) that differences in meanvalues are statistically significant. Values < 95% (marked with*) areconsidered not to be significant.


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long, compared to the second phase of pervasive alterationwith sealing of fractures. We therefore observe highlyaltered rocks on the core scale, as alteration was intense,but a modest alteration on the wireline scale as alterationprocesses leading to the sealing of fractures were delayed.

7. Summary and Conclusions

[38] Circulation of hydrothermal fluids through the flanksof mid-ocean ridges has important consequences for thecomposition of crust and water and considerably changesthe physical properties of the crust. In this study, wecompared petrophysical data on two scales (core and wire-line) from Hole U1301B drilled 3.5 Ma seafloor on theeastern flank of the Juan de Fuca Ridge and compared thesedata with data from other drillholes into upper oceanic crustranging from 5.9 to 167 Ma in basement age. Site U1301 islocated on a local buried basement high and until relativelyrecently basement outcrops around Site U1301 were ex-posed. Until most of the exposed basement in the area wasburied, the rate and extent of alteration were limited becausehydrothermal circulation extracted much of the lithosphericheat and kept basement fluid temperatures low. A subse-quent period of higher-temperature fluid circulation withmore restricted and less oxidative conditions followed whensedimentary burial increased and resulted in enhanced andpervasive alteration of crustal rocks. Our comparison showsthat this geological situation influences the physical prop-erties of the basement rocks at Site U1301. Bulk densityvalues measured on core samples from Hole U1301B tendtoward lower values for a given porosity than at other sitesinto young crust and also grain density is lower thanmeasured at other sites. Low bulk and grain density valuesindicate a stronger alteration and more enhanced replace-ment of original minerals by secondary phases thanexpected for the young basement age. Formation factorvalues derived from core measurements tend toward highervalues for a given porosity and suggest a tortuous micro-porosity probably caused by alteration. Core-scale P wavevelocity and bulk density values fall below global trendvalues predicted from petrophysical properties versus crustalage plots, confirming stronger alteration on the scale of coresamples. In contrast, wireline in situ measurements areconsistent with values predicted for young crust indicatingmodest alteration and open fractures. Particularly, the upper-most 200 m of basement in Hole U1301B are stronglyfractured and the major fractures, where most fluid flowoccurs, are still open. The results show that alterationprocesses in the oceanic crust can yield different intensitiesat the different scales of cores (microporosity) and wirelinelogs (fractures). The hydrothermal history of the crust sur-rounding Hole U1301B is likely to have been similar to thatof other holes drilled into local basement highs. Thesetectonic-volcanic features would have been some of the lastareas of regional basement exposure as the ridge flanksaccumulated sediments and became increasingly isolatedfrom the overlying ocean.

[39] Acknowledgments. A. Bartetzko was funded by the GermanScience Foundation (Ba 2196/2). A. Bartetzko thanks the Applied Geo-physics group of RWTH Aachen (Germany) for assistance with electricalconductivity measurements. A. Fisher was supported by National ScienceFoundation grant OCE-0550713. This research used data provided by the

Ocean Drilling Program (ODP) and the Integrated Ocean Drilling Program(IODP), and was supported with funding from the Joint OceanographicInstitutions (JOI) Inc., Projects T301A7 and T301B7. We thank S. M.Carbotte, D. Toomey, and an anonymous reviewer for their constructivecomments. This is RCOM publication number 0552.

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��A. Bartetzko, Research Center Ocean Margins, University of Bremen,

Leobener Strasse, 28356 Bremen, Germany. ([email protected])A. T. Fisher, Earth and Planetary Sciences Department and Institute for

Geophysics and Planetary Physics, University of California, 1156 High Street,Santa Cruz, CA 95064, USA. ([email protected])


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Physical properties of young (3.5 Ma) oceanic crust from ...afisher/CVpubs/pubs/BartetzkoFisherJGR_2008small.pdf · and water. Much of this ridge-flank circulation occurs at ... fluids